140 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Aphididae.— Unlike Hirschler (1912), Toth (1933) found in all the 

 genera and species of aphids studied by him symbionts in summer eggs 

 only after the cleavage of the nuclei. At a time after completion of the 

 blastoderm when the mycetom anlage fills the orifice between blastoderm 

 and the follicular cells, symbionts pass into the blastocoele. With the 

 inward growth of the mycetom anlage the yolk cells are pushed against 

 the anterior wall of the blastoderm, become flattened, and finally form a 

 mycetolemma, or envelope, of the mycetom. Simultaneously with the 

 invagination of the germ band the invasion of the symbionts into the 

 mycetom anlage takes place. The symbionts that infest the embryo are 

 those which normally are present in the mycetom of the mother aphid. 

 Toth (1933), confirming Klevenhusen, distinguished three principal types 

 of symbionts: a rounded form, another distinctly smaller coccus-hke 

 form, and a bacillus-hke form. Some species of aphids are infected with 

 but one kind of symbiont; others with either two or all three forms. The 

 first condition is most common. All three kinds are localized in different 

 mycetocysts which collectively form the mycetom. The large rounded 

 microorganisms are apparently the most primitive and occur without 

 exception in all aphids. They normally measure 2 to 4 microns. The 

 second form of symbiont is less than 1 micron in diameter but somewhat 

 variable both in size and shape. This form Toth (1933) found only in 

 species of the genus Stomaphis. The third form of symbiont is rod-hke, 

 usually less than 1 micron in diameter and varying in length from 4 to 

 20 microns depending on the host species. 



Since the symbionts from a single full-grown aphid must infect a 

 numerous progeny, rapid multiplication of these microorganisms is neces- 

 sary. Division of the cells is strikingly synchronous not only in a single 

 mycetocyte but often in the entire mycetom complex. With the division 

 of a symbiont there is a reduction in size, sometimes accompanied by a 

 loss of some of the gelatinous coat, followed by a gradual hypertrophy of 

 the daughter cells, restoring them to the normal size. 



A well-developed mycetom is always present in those aphids which 

 actively feed. The rudimentary, short-lived males of Stomaphis and of 

 Pemphigus filagi7ius, which do not feed, lack mycetom and symbionts. 

 In the rudimentary short-lived females of P. filaginus, which likewise do 

 not feed, the mycetom is reduced to a few mycetocytes; her symbionts 

 are completely utilized for infection of the young. These observations 

 of Toth (1933) seem to indicate that the symbionts have a nutritive 

 value, bearing out the experimental work of Koch on Sitodrepa and of 

 Aschner and Ries on Pediculidae. The relation of the mycetom to the 

 developing embryo is described in Part II in the section dealing with 

 the Aphididae. 



